1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-299726, filed on Nov. 3, 2006, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a mobile communications system having a plurality of base stations which control a plurality of cells, respectively. More particularly, the present invention relates to a mobility management method and device for selecting a cell to which a mobile station is connected.
2. Description of the Related Art
In a cellular mobile communications system having a plurality of cells controlled by a plurality of base stations, a mobile station generally exchanges data or control signals with base stations, detects a cell suitable for the mobile station to receive normal services (hereinafter, such a cell will be referred to as “suitable cell”), and thereby can camp on that cell. In the case where the mobile station cannot select a suitable cell to camp on, the mobile station can also select and camp on a cell from which the mobile station can receive limited services (hereinafter, such a cell will be referred to as “acceptable cell”). “Camping on a cell” means that a mobile station in an idle mode connects to a cell detected through cell selection/cell reselection, which will be described later.
For example, in the UMTS (Universal Mobile Telecommunications System) system, a mobile station that has camped on a cell receives the ID of the cell, the ID of a tracking area, and an undermentioned neighbor list through a broadcast channel (BCCH) from the base station (serving base station) controlling this cell, and monitors a paging channel (PCH). The mobile station also monitors pilot channels (CPICH) from neighboring cells intermittently. Neighboring cell information contained in the neighbor list typically includes the carrier frequencies of the neighboring cells and other system parameters. Therefore, by referring to the neighbor list, the mobile station searches for a cell to camp on while tuning in to the carrier frequency of each neighboring cell, whereby the mobile station can receive a downlink broadcast channel.
Moreover, when the mobile station has detected, based on the neighbor list, a better cell (generally, a cell exhibiting better radio quality) than the cell (hereinafter, referred to as “serving cell”) on which the mobile station is currently camping, then the mobile station camps on this newly detected cell as its serving cell. This operation is called “cell reselection”. The cell reselection is performed by a mobile station, based on some criterion provided from the network side. For example, when the mobile station has detected deterioration in the radio quality of its current serving cell, and if this state of deterioration continues for a period of time or longer, which is set on a reselection timer, then the mobile station performs cell reselection by using the neighboring cell information in the neighbor list. The cell reselection is generally performed through the following procedure. Specifically, while sequentially setting a carrier frequency according to the neighboring cell information stored in the neighbor list, the mobile station checks whether or not a neighboring cell having better radio quality than that of the current serving cell is detected. When such a cell having better radio quality is detected, the mobile station selects and camps on this cell having better radio quality as its new serving cell. As described above, since it is sufficient for the mobile station to check and detect the radio qualities only of the neighboring cells stored in the neighbor list, it is possible to select a cell having better radio quality at high speed. Thus, it is also possible to receive a paging message from the network with good reception probability. Additionally, the neighbor list is also utilized to measure the radio qualities of the neighboring cells at the time of handover.
If the mobile station cannot detect a suitable cell from among the cells on the neighbor list provided from the network side, the mobile station starts carrier sequential search, by which all possible carrier frequencies are sequentially searched, to find a suitable cell to camp on. The detection of a suitable cell by means of this carrier sequential search is called “cell selection” in the UMTS system. The cell selection includes initial cell selection and stored-information cell selection. In the initial cell selection, the mobile station, in ignorance of any carrier frequency information in advance, sequentially searches all existing frequency bands, thereby detecting a suitable cell. In the stored-information cell selection, the mobile station, having stored information about carrier frequencies in advance, carries out carrier sequential search of the frequency bands limited by the information, thereby detecting a suitable cell. The initial cell selection requires, for example, approximately twenty seconds to be complete because all radio frequency bands are searched. On the other hand, the stored-information cell selection enables higher-speed detection of a suitable cell because carrier sequential search is carried out in limited frequency bands. For example, the carrier sequential search according to the initial cell selection is carried out in the case where user equipment is used through an operator that uses a different band from any one of the frequency bands indicated by the stored carrier frequency information, as in the case where user equipment is powered on in a foreign country.
In the cell selection, in any case, the mobile station carries out sequential search of carrier frequencies. Therefore, the cell selection generally requires a longer time than the cell reselection, and services are interrupted during this period of time. In addition, a battery, by which user equipment is driven in general, is exhausted sooner because the sequential search consumes much power.
Procedures for the cell selection and for the cell reselection in the UMTS system are described in the following documents:                3GPP TS 25.304 “User Equipment (mobile station) procedures in idle mode and procedures for cell reselection in connected mode”;        3GPP TS 25.331 “Radio Resource Control (RRC); Protocol Specification (Release 6)”; and        3GPP TS 25.133 “Requirements for support of radio resource management (FDD)”.        
The neighbor list used in the cell reselection is sent to a mobile station from a base station on which the mobile station has camped. However, there is a possibility that not all cells are covered by the neighbor list, for example, in the case where a large number of small cells, called microcells or picocells, are deployed locally inside a building.
Japanese Patent Application Unexamined Publication No. 2001-526016 discloses a method by which a mobile station is allowed to connect to a suitable cell in a situation as described above where a cell that is not included in a neighbor list exists. Specifically, each base station in a neighborhood area transmits a single common list that includes all the measurement channels used in the neighborhood area. Thereby, a mobile station located in the neighborhood area can accomplish cell reselection using all the measurement channels.
In a mobile communications system provisioned with procedures for cell selection and cell reselection like the UMTS system, a mobile station performs cell selection when the mobile station moves into a cell that is not included in a neighbor list, because the mobile station cannot detect a suitable cell through cell reselection. In general, the mobile station performs stored-information cell selection and, if the mobile station cannot detect a suitable cell even through this stored-information cell selection, then performs initial cell selection. The larger the number of cells that are not included in a neighbor list as described above, the more frequently a mobile station performs cell selection. Accordingly, the problems arise that services are interrupted during cell selection, and that the power consumption is increased. Hereinafter, concerning these problems, a description will be given by using the accompanied drawings.
FIG. 1A is a schematic diagram of a cell arrangement, to describe an example of procedures for cell reselection and cell selection. FIG. 1B is a diagram showing a neighbor list for each cell, held on the network side. FIG. 1C is a diagram showing a neighbor list for each cell, provided from the network side and held by a mobile station. Note that the neighbor lists shown in FIG. 1B are maintained by, for example, a management server and the like on a network. In addition, although a neighbor list typically includes the carrier frequency of each neighboring cell and other system parameters as mentioned earlier, the neighbor lists shown in FIGS. 1B and 1C only include the ID of each neighboring cell in order to avoid complexity, and the illustration of the system parameters and the like included in each list is omitted.
In this example, it is assumed that cells C1-1, C1-2 and C1-3 are macrocells, each of which has a relatively large coverage area (radio area), and that a cell C2-1 is a microcell, which has a relatively small radio area. The microcell C2-1 is a radio area inside a building or the like, for example. It is assumed here that the microcell C2-1 is deployed inside a building located in the macrocell C1-2.
In such a cell arrangement, it is assumed that a mobile station moves from the cell C1-1 to the cell C1-2 and then, after further entering the microcell C2-1, moves back in the cell C1-2. First, when the mobile station is located in the cell C1-1, the cell C1-1 provides the mobile station with information about (a neighbor list of) the neighboring cells of the cell C1-1, {C1-2, C1-3}. Accordingly, when the mobile station leaves the cell C1-1, the mobile station monitors the radio qualities only of the neighboring cells C1-2 and C1-3 on this neighbor list, whereby the mobile station can camp on the cell C1-2, which is in the direction of the movement of the mobile station. That is, by referring to the neighbor list, the mobile station can camp on the cell C1-2 through cell reselection, without performing cell selection (carrier sequential search).
When the mobile station has camped on the cell C1-2, the cell C1-2 provides its neighbor list to the mobile station. Here, if the area covered by the microcell C2-1 is an indoor area inside a building, house or the like, there can be some cases where information about the microcell C2-1 is not included in the neighbor list of the cell C1-2, {C1-1}, as shown in FIG. 1B. If the information about the microcell C2-1 is not provided on the neighbor list of the cell C1-2, the mobile station, when moving from the cell C1-2 to the microcell C2-1, needs to carry out the cell selection procedure by means of carrier sequential search, in order to camp on the microcell C2-1.
The mobile station camps on the microcell C2-1 through the cell selection procedure. However, as shown in FIG. 1B, information about a neighboring cell of the microcell C2-1 is not provided to the mobile station. Accordingly, a neighbor list maintained by the mobile station as to the cell C2-1 on which the mobile station is currently camping is empty, { }, as shown in FIG. 1C. With the neighbor list in such an empty state, when the mobile station moves back to the cell C1-2 from the microcell C2-1, the mobile station needs to perform cell selection again to camp on the cell C1-2.
Such a mobile station moving between an indoor microcell and an outdoor macrocell needs to perform cell selection, which takes a longer time than cell reselection, every time the mobile station moves. While performing cell selection, the mobile station is neither able to be called nor able to submit a call-setting request, falling into a state incapable of receiving services.
To avoid such cell selection, a serving cell needs to provide information about all of its neighboring cells to a mobile station. However, it is not always possible to provide such information about all the neighboring cells, for the reasons of too many neighboring cells, a limit to the bandwidth for transmitting the neighbor list information, and the like.
Moreover, if the method described in Japanese Patent Application Unexamined Publication No. 2001-526016 is applied, each base station in a neighborhood area needs to transmit a common neighbor list including information about all the cells in the neighborhood area. This method has the problem that the loads on the network increase as the number of neighboring cells rises. In addition, if there is a limit to the bandwidth for transmitting the neighbor list, it is not always possible to provide information about all the neighboring cells as described above.